Method and use of nanoparticles to bind biocides in paints

ABSTRACT

This invention relates to the use of a combination of selected substances in paint to prevent the settlement and growth of different biofouling organisms with a reduced negative effect on the ecosystems compared to present methods.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is application claims priority from U.S. provisional applicationSer. No. 60/661,083 filed Mar. 11, 2005, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention herein relates to the method and use in an antifoulingpaint of an imidazole containing compound, such as Medetomidine, boundto metal nanoparticles to develop an efficient antifouling surface andimprove the performance of antifouling paints with regard to distributedfixation of the biocide in the paint matrix for even release and effectin hindering, for example, barnacle colonization. Other biocide systemscan be used with the same metal nano particle interaction for evenrelease in other paints than marine antifouling paint.

2. Description of the Related Art

The growth of biofouling organisms on underwater structures is a costlyand hazardous problem in both marine and freshwater applications. Thepresence of fouling organisms such as barnacles, algae, tube worms andthe like causes economic damage in various ways: for example attachmentto the hulls of ships reduces fuel efficiency and causes loss ofprofitable time because of the need to clean the hulls. Similarly, theattachment of these organisms to cooling water equipment decreases heatconductivity, which eventually reduces the cooling power of theequipment and drives up costs. Also other marine industries andinstallations, e.g. aqua culture equipment and oil/gas off-shoreinstallations and plants have significant problems with marinebiofouling.

Mechanical cleaning of marine surfaces has been introduced as analternative to toxides and biocides. Notably, water jet cleaning andmechanical cleaning using brushes are in use. The majority of thesemethods are, however, work-intensive and therefore expensive.

The most efficient antifouling paints have been “self-polishingcopolymer” paints based on a polymeric binder to which biocidalorganotin, in particular tributylin are chemically bound and from whichbiocidal organotin is gradually hydrolyzed by seawater as described forexample in UK patent GB-A-1457590. These organotin copolymer paintsprevent fouling by releasing the organotin compounds during hydrolysisof the polymer. The outermost paint layer becomes depleted of biocidesand is swept of the surface of the hull by the movement of the shipthrough water. Organotin copolymer paints also contain copper oxidepigment which is effective against biofouling against marine organismswhile the tributylin acts as a protection from slime and weed.

Paint containing organotin compounds, in particular tributylin haveproven to cause negative environmental consequences, harming sea life,causing deformations in oysters and sex changes in whelks. It have beennoted that organotin compounds are degraded slowly, and as a consequencethese compounds have been accumulated in the sediments in localizedareas. Several countries and international organizations have thereforeintroduced restrictions and prohibitions over their use and furtherrestrictions are expected. Sale and application of tributylinantifouling is to cease, under the International Maritime Organization(IMO) Antifouling System Convention agreed in October 2001. The treatycalls for the ban on application from 1 Jan. 2003 and total prohibitionon hulls by 1 Jan. 2008.

With the recent restrictions on the use of these toxic coatings in manycountries, the boat and ship owners have fallen back to the technicallyinferior but less toxic copper oxide based coatings. The life of copperoxide based coatings rarely exceeds 2 years in normal fouling conditionscompared to 5 years with self-polishing tributylin. Dissatisfactionexisted because copper oxide based coatings did not satisfy the boat andship operators and owners. Neither did it satisfy environmentalprotection organizations because of its toxicity to the environment.When the copper compounds are used in reduced concentrations forecological reasons these paints need however booster biocides againstbarnacles and algae to achieve performance acceptable for ship ownersand other types of marine industries.

Recent progress within the field of self-polishing paints includes theuse of a zinc acrylate copolymer utilizing ion exchange as the releasemechanism.

Concern for the possible effects of antifoulant toxicants on theenvironment has encouraged the development and use of systems whichattempt to control fouling through surface modifications; for example,prevention of attachment through the use of silicone or fluorinecontaining polymers having non-stick or release properties, describedfor example in the following patent documents WO-0014166A1, US92105410,JP53113014, US92847401, DE2752773, EP874032A2, and EP 885938A2. It hasbeen shown that these paints tend to be fragile, resulting in crackingand peeling of the surface.

A new alternative technology was introduced early in the 1990s. Althoughthis was also said to be self-polishing technology, the process toobtain this was no longer through hydrolysis of a polymer. Insteadcombinations of different water sensitive and partly water solublebinders such as rosin, alone or mixed with acrylates as described ine.g. European patent EP0289481, EP526441 were used. The experience hasshown that these paints have not been able to provide the same high andreliable performance as the hydrolyzing organotin-based paints.

Lately new polymers have been developed, based on the same principles asthe organotin polymers, i.e. hydrolysis of an insoluble polymer toprovide a slightly water soluble product. Among these are e.g. theself-polishing polymers described in WO8402915. Instead of incorporatingorganotin groups in the polymer chain, this describes the incorporationof organosilyl groups. Experience has shown that these paints have manyof the properties associated with the organotin copolymer technology.However, it has also been found that over a long period cracking andpeeling on the surface these paints may occur. This is caused by theleaching of soluble components, resulting in the formation of a residuallayer that has a different composition than the original paint.

An approach to solve this has been to modify the silyl polymer withdifferent co-monomers, described in EP0646630, EP1016681 and EP1127902.Another approach has been to include fibres to strengthen and increasethe cohesive strength in the whole paint and particularly all theresidual layer formed as described in WO0077102. A third approach hasbeen to develop a paint wherein mixtures of organosilyl copolymers androsin have been used to reduce the build up of this residual layer. Thishas been described in EP0802243. The use of low molecular plasticizers,more specifically chloroparaffines, has also been employed. This isdescribed in EP0775733.

Along the Swedish west coast as well as along the coasts of the NorthAtlantic Ocean, barnacles and algae are an economic and technicalproblem. The fully grown barnacle is a stationary crustacean,characterized by a centimeter sized cone shape and enclosing layers ofcalcinous plates. The mechanical strength of the animal's attachment tosolid surfaces is very high, which is why it is difficult tomechanically remove barnacles from solid surface. The animal undergoesdifferent development stages as free-swimming larvae, where the lastlarva stage is referred to as the cyprid stage. The cyprid screens solidsurfaces suitable for settling with the help of a nervous protuberance.In connection with settling, the “settling-glue” referred to as balanuscement is secreted from specialized glands localized to the protuberanceand the animal thereby settles to the solid surface. After settlementthe animal undergoes a metamorphosis into an adult and stationaryanimal. When using an old copper leaking paint, with high concentrationof copper, one of the first organisms to foul is barnacles.

Algae are also relatively insensitive to copper and the amount ofleaking copper needed to inhibit fouling of algae is high. Therefore,copper-containing marine antifouling paints are “boosted” by somemanufacturers with more specific algicides. The algicides inhibit thezoospores to attach or inhibit the photosynthesis. Both methods give theresult of reduced algae fouling.

A future antifouling paint, boosted with a biocide, should act with highspecificity i.e. only target fouling organisms being affected, leavingother marine mechanisms unharmed. The paint should also be designed toattain a controlled release of the active substance. An efficientapproach to accomplish a controlled release is by the formation of abond to a large molecule. Due to a large size and low mobility of alarge molecule the biocide diffusion through the paint film can berestricted and thereby have a release rate which is only dependent onthe polishing rate of the self-polishing paint. Furthermore thebiodegradation of the antifouling agent is another important aspect inorder to prevent accumulation in water and sediments and thus affectingthe marine environment rather than the target biofouling organism alone.

Several compounds have been presented with antifouling activity. Amongthose compounds are pharmacological agents with known pharmacologicalprofiles in vertebrates. It has been reported that a selection ofpharmacological compounds, that act upon serotonin and dopamineneurotransmitters has the ability to either impede or promote theattachment of barnacles. Serotonine antagonists, such as Cyproheptadineand Ketanserin, and dopamine agonists, such as R (−)-NPA and(+)-Bromocriptine, have exhibited inhibitory properties. Anotherpharmacological agent that has proven to be an efficient inhibitor withregards to barnacle settlement is the highly selectivealpha2-adrenoreceptor agonist Medetomidine or(S,R)-4(5)-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole. The larvalsettlement is impeded already at low concentrations, 1 nM to 10 nM.Medetomidine belongs to a new class of alpha2-receptor agonistscontaining a 4-substituted imidazole ring with, high selectivity towards2-adrenoreceptors. Receptors affected by catecholamineneurotransmitters, such as norepinephrine and epinephrine, are termedadrenergic receptors (or adrenoceptors) and can be divided into alpha-and beta-subclasses. The alpha2-adrenoreceptors are involved in theautoinhibitory mechanism of neurotransmitter release and play asignificant part in the regulation of hypertension (high bloodpressure), bradycardia (reduced heartbeat rate) and even regulation ofalertness and analgesia (reduced sensitivity to pain). Medetomidine hasbeen studied in human clinical trials and has also been used asanaesthetics for animals with the (S)-enantiomer, Dexmedetomidine, beingthe active component.

Nanoparticles are nanometer-sized metallic and semiconducting particlesthat have recently been the subject of extensive research in the fieldof nanoscale materials. Nanoparticles have potential applications inmany diverse fields. These applications include: nanoscale electronicdevices, multifunctional catalysts, chemical sensors, and manybiological applications such as biosensors, biological assays,transfection of organisms using gene-gun technology, and drug delivery.

Two important factors cause the properties of nanomaterials to differsignificantly from other materials: increased relative surface area, andquantum effects. These factors can change or enhance properties such asreactivity, strength and electrical characteristics. As a particledecreases in size, a greater proportion of atoms are found at thesurface compared to those inside. For example, a particle of size 30 mmhas 5% of its atoms on its surface, at 10 nm 20% of its atoms, and at 3nm 50% of its atoms. Thus nanoparticles have a much greater surface areaper unit mass compared with larger particles. As growth and catalyticchemical reactions occur at surfaces, this means that a given mass ofmaterial in nanoparticulate form will be much more reactive than thesame mass of material made up of larger particles (ref “Nanoscience andnanotechnologies: opportunities and uncertainties”, Jul. 29, 2004, Royalacademy of Engineering, UK).

Also the use of nanoparticles in paints, for antifouling and other uses,has been previously discussed but then in order to change the structureof the painted surface to be able to be for example thinner, or smootherfor less fouling on marine structures (ref “Environmental applicationsand impacts of nanotechnology 8 Dec. 2003, proceedings from the Royalacademy of Engineering, UK) or rougher surfaces for less fouling (refhttp://innovation.im-boot.org/modules.php?name=News&file=article&sid=129)but not the concept of using nanoparticles for the specific bindings ofbiocides disclosed in the present invention.

It is therefore an object of the invention to provide a method andproduct for use in antifouling products such as paints, usingnanoparticles to bind the biocides. Other objects and advantages will bemore fully apparent from the following disclosure and appended claims.

SUMMARY OF THE INVENTION

The invention herein relates to the method and use in an antifoulingpaint of an imidazole containing compound, such as Medetomidine, boundto metal nanoparticles that specifically and efficiently impedesettlement of for example barnacles on aquatic structures. Medetomidinewas surprisingly found to adsorb strongly to metal nanoparticles, afeature of particular interest in attempts to develop an efficientantifouling surface and improve the performance of antifouling paintswith regard to distributed fixation of the biocide in the paint matrixfor even release and effect in hindering for example barnaclecolonization. Other biocide systems can be used with the same metal nanoparticle interaction for even release in other paints than marineantifouling paint.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the chemical structures of the investigated antifoulingagents: a) Chlorothalonil, b) Dichlofluanid(N′-dimethyl-N-phenylsuphamide), c) SeaNine (SeaNine™(4,5-dichloro-2-n-octyl-3(2H)-isothiazolone), Rohm and Haas Company,Philadelphia, Pa.), d) Irgarol(2-methylthio-4-tert-butylamino-6-cyclopropylamino-s-triziane), e)Diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea), DuPont AgriculturalProducts Wilmington, Del., and f) Tolylfluanid(N-(Dichlorofluoromethylthio)-N′,N′-dimethyl-N-p-tolylsufamide).

FIG. 2 a) and FIG. 2 b) show plots of the fraction adsorbed antifoulingagent (Medetomidine, Chlorothalonil, Dichlofluanid, SeaNine, Irgarol,Diuron, and Tolylfluanid) versus surface area (m²) of FIG. 2 a) beingZnO and FIG. 2 b) being CuO nanoparticles, in o-xylene.

FIG. 3 is a plot of the fraction adsorbed Medetomidine versus surfacearea (m²) of ZnO, CuO, Al₂O₃, MgO, TiO₂, Co₃O₄ nanoparticles and CuO (5μm), in o-xylene.

FIG. 4 is a plot of the fraction adsorbed Medetomidine plotted versussurface area (m²) of ZnO and CuO, nanoparticles in o-xylene,acetonitrile and butanol.

FIG. 5 is a plot of the amount released Medetomidine (ng) versus time(weeks) for CuO and ZnO nanoparticles-Medetomidine modified paints andMedetomidine modified marine paint.

FIG. 6 is a plot of the amount released Medetomidine and SeaNine (ng)versus time (weeks) for CuO and ZnO nanoparticles-Medetomidine ornanoparticles-SeaNine and Medetomidine, SeaNine modified marine paints.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF

Recent studies, not yet published, have shown that nanoparticles, suchas copper(II) and zinc(II) Oxide (33 and 53 nm in diameter,respectively) can be utilized in order to sustain a controlled releaseof, for example, the antifouling agent Medetomidine. The use ofnanoparticles is of interest due to their tremendously large specificsurface area (ratio between surface area and particle volume). The CuOand ZnO particle generates specific surface areas of 29 and 21 m²·g−1,respectively.

When mixing CuO and ZnO nanoparticles with Medetomidine and otherantifouling agents such as Chlorothalonil, Dichlofluanid, SeaNine,Irgarol, Diuron, and Tolylfluanid, in o-xylene, a considerabledifference in interaction is observed. Large fractions of Medetomidineare adsorbed already at low particle concentrations, in particular whenemploying ZnO. This allows the possibility to design a paint systemcontaining low amounts of both Medetomidine and nanoparticles in orderto restrict the diffusional motion of antifouling agent through thepaint film. The adsorption of Medetomidine is highly favoured incomparison to the other antifouling agents listed above. The antifoulingagents listed above exhibit one common feature; nitrogen is present inall compounds either as secondary or tertiary amine, nitrile group or ina heterocyclic ring. However, the study implies that the imidazolemoiety of Medetomidine has the optimal geometry for adsorption onto theparticle surfaces.

In order to investigate the significance of a large surface area, theinteraction of Medetomidine and various metal oxide nanoparticles (ZnO,CuO, Al₂O₃, MgO, TiO₂, Co₃O₄) and a micrometer sized particle, CuO (5μm) has been studied. When substituting a nanoparticle with a micrometersized particle, the Medetomidine adsorption appears to be negligible.These results show the importance of large surface areas with regard toMedetomidine adsorption onto particle surfaces.

One object of this invention is to create an antifouling methodrequiring decreased biocide dose which is ecologically and economicallyadvantageous. In order to improve the performance and to reduce theeffect on the environment, it is important to have a proper control ofthe release of the antifouling substance from the paint film. TheMedetomidine molecule bound to nanosized metal oxide is a compound thatleaks out of the paint into water in a controlled fashion. TheMedetomidine molecule bound to nanosized metal oxide has excellentdispersion stability because of its large size, compared to theMedetomidine particle alone. By size property the Medetomidine-metaloxide particles are stationary in a SPC paint film and don't leak outinto the water. As a consequence the concentration of antifoulingparticles in the paint film remains homologous during “lifetime”.

Metal nanoparticles provide a great number of binding sites forMedetomidine and a high amount of Medetomidine can be bound. As a resultthe concentration of Medetomidine will be equal in the entire paintfilm. Thereby the desorbtion will be at a uniform level and a minimumamount of Medetomidine will be needed to achieve antifouling effects.Another consequence is that the total surface area of nanoparticles isenough to adsorb all of the Medetomidine and there will be no waste ofthe biocide.

When exposed to water the Medetomidine in the surface layer dissociatesfrom the metal oxide and desorbs from the surface. A surface-activecompound in antifouling paint is thus likely to have a greater impact onsettlements of barnacle larvae than a compound leaking out of the paintinto the water since surface activity will increase the concentrationclose to the surface.

Due to a broad surface area relative to its size metal nanoparticlesprovide a great number of binding sites for biocides. As a consequencethe amount of metal oxide can be decreased and thereby the negativeeffect on the environment achieved from metal oxides.

Medetomidine in accordance with the invention is comparatively harmlesscompared with the toxic substances presently used in ship hull paints.Indeed, Medetomidine in accordance with the invention is so harmlessthat it is approved as pharmaceutical preparations for internal use.Medetomidine is also biodegradable so there is less bioaccumulation ofthis substance, and it is therefore ecologically safer than manyexisting antifouling compounds. Other imidazole containing biocides,such as for example anti fungal Micronazole can be used.

Example 1 Study of the Interaction Between Various Nano Particles andBiocides Materials and Methods

To 50 ml of the solvent o-xylene, the biocide was added at a totalconcentration of 50 mM. Nano-particles (Sigma-Aldrich Sweden AB,Stockholm, Sweden) were then added in relevant concentrations accordingto the test protocol and after each addition the concentration ofnon-adsorbed Medetomidine (Orion Pharma, Helsinki, Finland) was measuredwith a standard HPLC-UV technique. The UV-absorbance maximum ofMedetomidine was, from the literature, found to be 220 nm. Prior to theHPLC analysis the UV-absorbance maximum was investigated in order toverify the value given from the literature using a UV-spectrometer (GBC920 UV/visible spectrometer, Scientific Equipments Ltd., Victoria,Australia).

The samples were analysed on a HPLC-UV system consisting of aMerck-Hitachi L-6200 pump (Merck-Hitachi, Darmstadt, Germany), a SupelcoDiscovery® (Sigma-Aldrich Sweden AB, Stockholm, Sweden) C18 (25 cm×4.6mm, 5 m) column fitted with a prefilter (0.5 μm) and a Spectra-PhysicsSpectra 100 UV (Spectra-Physics Inc, Irvine Calif., USA) operating at220 nm. The mobile phase was MilliQ-water:Acetonitrile (0.1% TFA v/v(mobile phase A): 0.1% TFA v/v (mobile phase B)) run over a gradient (6%B for two min then an increase up to 60% B in 15 min, a further increaseup to 100% B in 3 min and then isocratic for 3 min before returning tothe starting values in 2 min) at a flow rate of 10 ml/min. The peakseparation was monitored by UV (220 nm). Manual injections of 100micro-liter were made and the data was collected and integrated usingthe Millenium software (version 3.20, 1999)(Waters Inc, Milford Mass.,USA).

Results

When mixing CuO and ZnO nanoparticles with Medetomidine and otherantifouling agents such as Chlorothalonil, Dichlofluanid, SeaNine,Irgarol, Diuron, and Tolylfluanid, in o-xylene (see FIGS. 1, 2 a) and 2b), a considerable difference in interaction is observed. Largefractions of Medetomidine are adsorbed already at low particleconcentrations, in particular when employing ZnO. This allows thepossibility to design a paint system containing low amounts of bothMedetomidine and nanoparticles in order to restrict the diffusionalmotion of antifouling agent through the paint film. The adsorption ofMedetomidine is highly favoured in comparison to the other antifoulingagents listed above. The antifouling agents listed above exhibit onecommon feature; nitrogen is present in all compounds either as secondaryor tertiary amine, nitrile group or in a heterocyclic ring. However, thestudy implies that the imidazole moiety of Medetomidine has the optimalgeometry for adsorption onto the particle surfaces.

It should be noted that while Medetomidine exhibited the bestadsorption, some of the other compounds, particularly SeaNine, as wellas diuron and igarol, also exhibited adsorption in this study.

Example 2 Study of the Interaction Between Various Sizes of Particlesand Biocides Materials and Methods

The different nanoparticles used (ZnO, CuO, Al₂O₃, MgO, TiO₂, Co₃O₄)were bought from Sigma (Sigma-Aldrich Sweden AB, Stockholm, Sweden) andused without further purification. 50 ml of o-xylene was added to abeaker to which 50 mM of Medetomidine (Orion Pharma, Helsinki, Finland)was added. Nanoparticles were then added and after each addition theamount of free Medetomidine was measured with HPLC-UV (as in example 1above).

Results

In order to investigate the significance of a large surface area, theinteraction of Medetomidine and various metal oxide nanoparticles (ZnO,CuO, Al₂O₃, MgO, TiO₂, Co₃O₄) and a micrometer sized particle, CuO (5μm) has been studied (see FIGS. 3 and 4). When substituting ananoparticle with a micrometer sized particle the Medetomidineadsorption appears to be negligible. These results show the importanceof large surface areas with regard to Medetomidine adsorption ontoparticle surfaces.

Example 3 Study of the Release Rate of a Biocide from Nano ParticlesMaterials and Methods

The paint chosen for these studies was a self-polishing paint withxylene as the solvent, the SPC Lefant marine paint from Lotrec AB(Lindingo, Sweden), To one litre paint a 50 ml solution containing 10 gof nano particles (CuO and ZnO) (Sigma-Aldrich Sweden AB, Stockholm,Sweden) and surface adsorbed Medetomidine (Orion Pharma, Helsinki,Finland) or SeaNine (Rhome & Haas, Philadelphia, Pa., USA) was added andmixed under vigorous stirring for five minutes. Triplicate samples wereprepared and painted using a paint applicator in order to assure ahomogeneously thick paint film which in this case was 200 micrometer.The painted plates were 10×10 cm and placed in artificial seawater for aperiod of 8 weeks.

Results

The Medetomidine-nanoparticle interaction (Medetomidine-CuO andMedetomidine-ZnO) has also been employed for release rate studies (seeFIG. 5). After eight weeks the Medetomidine-nanoparticle modified paintsgenerated a 20% decrease in released amount of Medetomidine incomparison to the paint with Medetomidine as single additive. Theresults for the SeaNine-nanoparticle interaction are showed in FIG. 6,which demonstrates a similar decrease in release as with theMedetomedine-nanoparticle combination.

Example 4 Manufacture of a Paint for Anti-Fouling Using Nano Particlesto Bind a Biocide

As one example of a Medetomidine modified paint a typical paint chosenfor these studies contained Xylene as the main solvent (the SPC Lefantmarine paint from Lotrec AB, Lindingo, Sweden), In order to prepare thepaint containing both nanoparticles and biocides these two componentswere first mixed in a solvent allowing for very strong adsorption ofbiocide, e.g. xylene. Typically, 10 gram of nanoparticle was stirred in50 ml of xylene (using a simple magnetic stirrer) together with amaximum of 10% excess of non-adsorbed Medetomidine, After completeadsorption (typically a few minutes of mixing) the solution was slowlyadded into the paint under quite vigorous stirring (Stirring was carriedwith a propeller-like shearer working at approximately 0.5-2 Hz) untilthe paint became homogeneous which took typically 5-10 minutes dependingon the stirring rate.

Nanoparticles were bought from Sigma (Sigma-Aldrich Sweden AB,Stockholm, Sweden) and used without further purification Medetomidinewas bought from Orion Pharma, Helsinki, Finland.

While the invention has been described with reference to specificembodiments, it will be appreciated that numerous variations,modifications, and embodiments are possible, and accordingly, all suchvariations, modifications, and embodiments are to be regarded as beingwithin the spirit and scope of the invention.

1. (canceled)
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 10. (canceled) 11.A product for preventing marine biofouling of a substrate by a marinebiofouling organism, comprising a protective paint containing animidazole-containing compound bound to metal oxide nanoparticles. 12.(canceled)
 13. The product for preventing marine biofouling according toclaim 11, wherein the metal oxide nanoparticles are selected from thegroup consisting of CuO, ZnO, TiO₂, Al₂O₃, SiO₂, and MgO.
 14. Theproduct for preventing marine biofouling according to claim 13, whereinthe metal oxide nanoparticles are CuO.
 15. The product for preventingmarine biofouling according to claim 13, wherein the metal oxidenanoparticles are ZnO.
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 18. (canceled) 19.(canceled)
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